1. Field of the Invention
The present invention relates to a servocontrol process for a servovalve which can be servocontrolled in terms of flow rate or pressure. It applies in particular to hydraulic servovalves used in vehicle automatic braking or guidance systems.
2. Discussion of the Background
Such servovalves include a hydraulic slide provided with return means and moved by a rotary actuator such as an electric motor; the return means keep the slide in a neutral position from which the slide can be translated in two opposite directions so as to obtain two working positions, corresponding for example to the pressurizing or venting to tank of the circuit.
Such servovalves include two servocontrol loops: one, which is called the flow rate control loop, cuts in when the servovalve is intended to meter a flow rate and the other, which is called the pressure loop, cuts in when the servovalve is intended to control a pressure inside a volume, a brake, a jack, etc.
Such servovalves are described in particular in French Patent Application No. 92 02843 of Mar. 10, 1992 and No. 94 07571 of Jun. 21, 1994 from the Applicant.
In this autosystem, the loops employ: a sensor which measures the position of the motor linked to the slide of the servovalve, in the case of flow rate servocontrol; one or two pressure sensors which measure the pressure in one or two volumes, in the case of pressure servocontrol. The measurements are compared with position set-points for the flow rate loop and pressure set-points for the pressure loop or loops.
In order to deal with the matter optimally, that is to say by effecting a compromise between gain, phase and stability of the servocontrols, use is made of the knowhow from techniques in the field of automation, generally relating to servocontrol loops, in which PID (Proportional Integral Derivative) functions are employed: linear type processing.
In the case of the control of servovalve loops, the use of a proportional derivative control is not always obvious. The reason for this is that for small signals it would be necessary for the loops to have a first gain and, for large signals, another gain. A particular difficulty is then faced, namely of adding a particular type of control command to the linear type controls.
The mechanical elements which prompt particular consideration of control command are linked with the jet effect, and also with the fact that the electric control motor possesses considerable self-inductance and also inertia; this is due to the power which it must develop in order to counter the forces generated by the jet effect.
Moreover, when it is necessary to work on considerable signal deviations, the regulating loops determine power control levels for the motor which are insufficient for the optimal speeds to be obtained under transient conditions. In this case it is therefore necessary to append a state control module which determines the current to be applied to the motor and, consequently, the torque to be imposed on the motor. Control of the power is carried out in that portion of the hardware consisting of the transistor bridge controlling the motor and the duty ratio for the control of this bridge. The duty ratio varies in relation to the software which imposes the state control.
When the slide is actuated, for example from its position of venting to tank, to its position of pressurization, it traverses a dead zone for which there is no link in the circuits to be controlled and, consequently, no action of the hydraulic circuit takes place. This dead zone causes a hysteresis which reduces performance especially in the case of the actuation of devices such as brakes.